Gasket materials are used in many chemically corrosive and high temperature environments leading to their constant chemical and thermal degradation and required maintenance and replacement. Such applications of these gaskets include electrolytic cells (including tank-type cells, filter press cells, and diaphragm cells), plate and frame heat exchangers and tank car dome lids.
There has been a substantial effort in both developing and commercializing gaskets that are capable of withstanding these harsh environments. Gaskets presently available for the above-mentioned applications are generally made of rubber or other synthetic materials such as ethylene-propylene-diene rubber (EPDM), Viton, or nitrile rubber because of their resiliency, conformability, and good low clamp load sealability. These gaskets operate effectively for a brief period but then rapidly deteriorate in very hot and corrosive environments. As the gaskets deteriorate, they leak and require replacement.
Efforts to improve gasket construction to enable successful long-term use in high temperature and corrosive environments have focused on the use of materials such as polytetrafluoroethylene and particularly expanded polytetrafluoroethylene because of its ability to withstand corrosive environments and high temperatures. A gasket made entirely of polytetrafluoroethylene is typically not practical however because of high costs and its mechanical properties render it impractical for use by itself as a seal in many applications.
Attempts have been made to combine a variety of materials with polytetrafluoroethylene to enhance long-term sealability in chemically corrosive environments. To date however, these efforts have resulted in products that have drawbacks and complications which limit their usefulness.
U.S. Pat. No. 4,344,633 describes a multi-layer gasket having an outboard layer of material with a hardness coefficient of 40-70% (ASTM Spec. D-395-69) in combination with a separate inboard layer of material which is corrosion resistant, non-contaminating, and stable upon contact with the cell anolyte. The inboard layer is described to be a rope or strip of low density expanded polytetrafluoroethylene "in the `GORE-TEX` form."
There are several problems with the product described in U.S. Pat. No. 4,344,633 thereby limiting its use in sealing applications with electrolytic cell parts. Equipment must be specifically designed to employ this two-part gasket as the flange of the cell must be wide enough to support the inboard barrier seal. Also, the inboard barrier seal is comprised of low density expanded polytetrafluoroethylene which provides a liquid tight seal but is still permeable to corrosive vapors that pass through the barrier seal to attack the rubber seal.
European Patent Application 0117085 describes a method of bonding a layer of fluoropolymeric material to the surface of an organic polymer material. The preferred fluoropolymeric material is expanded and of low density form and is most preferably expanded polytetrafluoroethylene. The process described in the application includes placing the expanded low density fluoropolymer into a mold, charging an organic polymer to the mold so that is contacts the fluoropolymer and subjecting them both to elevated temperatures and pressures so that a bond is created. Similar to U.S. Pat. No. 4,344,633, these gaskets utilize low density expanded polytetrafluoroethylene which is permeable to corrosive vapors and gases when there is insufficient compression along all points of the gasket.
Additional problems with the above-mentioned references include the shim effect created by the construction of a separate polymeric material causing incomplete densification of the low density expanded polytetrafluoroethylene. If the expanded polytetrafluoroethylene is not compressed to above a density of 1.7 g/cc, it will be permeable to corrosive vapors and therefore not function properly.
A separate barrier material will allow leakage past it during vibration and thermal cycling when load--unload conditions exist.
Also a problem exists with the above described reference when the polytetrafluoroethylene fails to match exactly the conformation of the elastomer which may result in leakage and attack of the elastomer.